In order to provide electrical isolation, e.g., for safety considerations, most switching DC-DC power converters employ conventional optocouplers. FIG. 1 illustrates a prior art optocoupler circuit 100. A steady DC voltage V.sub.IN received from the output of a main switching converter (not shown) is scaled by a resistor network 101 and 103 and is compared to a reference voltage V.sub.REF via a high gain amplifier 107. Grounding point 102 comprises the power return path. The high gain amplifier 107 compares the scaled V.sub.IN and V.sub.REF and outputs an error signal voltage V.sub.ERR, representing the difference between V.sub.IN and V.sub.REF. The error signal voltage V.sub.ERR drives an LED 111, causing LED 111 to emit light across isolation barrier 113 to a photo-transistor 119. Grounding point 122 comprises the power return path for this side of the circuit. In a known manner, photo-transistor 119 converts the light emitted from LED 111 back to a current signal representing the difference between the scaled V.sub.IN and V.sub.REF, which is converted by resistor 117 back to a voltage representing the error signal voltage V.sub.ERR. Resister 105 converts the bias current input to or output from high gain amplifier 107, thereby balancing any input bias-voltage imbalance.
The optocoupler circuit 100 of FIG. 1 provides isolation across isolation barrier 113; however, it operates with a restricted temperature range because the semi-conductor junction materials of photo-transistor 119 can only withstand temperatures between -20.degree. centigrade and 95.degree. centigrade, thereby limiting the dynamic range of the circuit. In addition, since the light emitted by light emitting diodes such as LED 111 is relatively weak in intensity, the isolation barrier 113 between LED 111 and photo-transistor 119 must be kept relatively small. Due to the close proximity of LED 111 with respect to photo-transistor 119, capacitive coupling can occur between the two devices, thereby introducing AC coupling between the two devices and degrading the isolation that they provide.
It is also well known to utilize transformers to provide isolation between two electrical circuits so as to isolate a source of relatively high voltage that powers a device from low voltage devices and/or from a user of the isolated device. For example, isolation transformer are commonly used in medical equipment, such as temperature monitors, electro-cardiograms, oximeters, or invasive blood pressure monitors which include sensors which are in contact with the patient. U.S. Pat. No. 5,615,091, for example, incorporated fully herein by reference, is directed to an isolation transformer for medical equipment.
Non-isolated current sampling voltage summing circuits are also known. FIG. 2 illustrates a prior art current sampling voltage summing circuit 200. As shown in FIG. 2, a resistor-divider comprising resistors 201 and 203 scales a steady DC voltage V.sub.IN from a main switching converter and provides an input to a comparator, e.g. high gain amplifier 207. Grounding points 202 comprise the power return path for this side of the circuit. High gain amplifier 207 compares this input with a reference voltage V.sub.REF input via resistor 205, just as in FIG. 1. The output V.sub.ERR of high gain amplifier 207 is an error voltage signal which is applied to the base of transistor 223 via resistor 221. Transistor 223 acts as a voltage follower, since the error signal voltage V.sub.ERR will go across the base-emitter junction of transistor 223 and "sit" on top of the emitter. Thus, the emitter voltage of transistor 223 is the sum of the base-emitter junction voltage of the transistor 223 plus the error voltage V.sub.ERR, in volts.
A power transformer T1 having a primary winding 227 and a secondary winding 229 is switchable between an energized and a de-energized state by switching transistor 225. Grounding point 222 comprises the power return path for this side of the circuit. In a known manner, a non-isolated current sensing/sampling block 240 yields a current output kI.sub.P that is a scaled version of primary current I.sub.P at current input node 228 with the scaling factor k. The output current, kI.sub.P passes an emitter resistor 209 and produces a pulsating voltage V.sub.SENSE. This pulsating voltage V.sub.SENSE is added to the error voltage sitting at the emitter of transistor 223, and the sum of these two voltages presents itself as a non-isolated feedback signal used in a pulse width modulator. By comparing the non-isolated feedback signal with another known reference voltage, a driving pulse with variable time duration (width) is provided for switching transistor 225. However, due to the non-isolated nature of current sensing/sampling block 240, grounding point 202 and grounding point 222 are in essence the same.
The magnetic coupling circuit 200 of FIG. 2 has certain deficiencies. It does not provide isolation and it must "overcome" the base-emitter junction voltage of transistor 223; thus it is unable to handle low-level signals. For example, if the primary current I.sub.P is relatively small and the sampled current output kI.sub.P multiplied by the ohmic value of the sensing resistor 209 is not large enough to overcome the base-emitter junction of transistor 223, the circuit will not function because the circuit is, in effect, an open circuit. This will cause the control loop to be opened rendering it unable to control the converter output V.sub.IN. To properly function, the circuit must function at all times.